1. Field of the Invention
This invention relates to bearings, and more particularly to a maintenance free bearing having a lubricious wear layer that is resistant to creep.
2. Background Information
Maintenance-free sliding bearings comprising a metal support and a plastic layer are known. Such bearings provide convenient means for rotatably, pivotably or slidably fastening multiple members to one another in a maintenance free manner. Applications for such bearings include those that utilize continuous rotational movement such as journals for supporting a driven shaft. These bearings are also suitable for applications that employ repeated pivotal movement such as automotive door hinges, door checks, brake and accelerator pedals. Additional applications include those that utilize repeated reciprocal movement such as automotive shock absorbers and struts. These bearings may also be used in lighter duty applications such as multiple bar linkages commonly utilized in the automotive industry for trunk deck lid and hood hinges. Such maintenance free bearings may comprise a variety of configurations, such as, for example, bushes or journal bearings, thrust bearings or washers, locating pads, valve port plates, and wearing components for a variety of mechanisms.
One such bearing in particular is referred to herein as a "DU" bearing available from The Glacier Metal Company Limited, Argule House, Joel Street, Northwood Hills, Middlesex HA6 1LN, England. The DU bearing consists of a composite material in which a porous bronze layer is bonded to a metal backing. The porous bronze layer is impregnated with a polymer such as PTFE (polytetrafluoroethylene), with a top layer or lining of polymer disposed thereon.
One aspect of this construction is that the ratio of polymer to bronze tends to change with depth, with the bronze being relatively more concentrated closer to the metal backing. This provides a reduced concentration of low friction material close to the backing metal. Thus, the coefficient of friction tends to disadvantageously vary (increase) over the bearing life. An additional drawback of this concentration gradient is that any operation that removes material from the bearing layer, such as the common practices of boring, broaching or burnishing the bearings to size after installation, generally cannot be accomplished without a reduction in bearing performance.
A bearing developed to overcome these limitations, sold under the designation "Norglide", is available from Norton Pampus, GmbH, of Willich, Germany. Norglide comprises a thin sheet of bearing material, such as, for example, a PTFE compound, bonded onto steel backing using high temperature thermoplastic films, (eg. PFA and ETFE) heat and pressure. Since the bearing layer is fabricated as a discrete sheet, rather than a dispersion as in the case of the aforementioned DU bearings, the Norglide bearing layer is homogeneous. This aspect advantageously provides a coefficient of friction that remains nominally constant throughout the bearing life. Moreover, the coefficient of friction of the Norglide bearing may be lower than other prior art bearings due to the ability to utilize reduced filler content or fillers that have less negative impact on the coefficient of friction. For example, the Norglide bearing may utilize graphite filler rather than bronze as discussed hereinabove. Also, such homogeneity enables the bearing surface of Norglide bearings to be bored, broached or burnished to size after installation nominally without reducing the performance thereof.
This construction, however, is not without limitations. In particular, the relatively high thickness of the PTFE compound and the low filler content tend to enable the bearing layer to creep or bed in under heavy stress. Moreover, the bearing layer may tend to delaminate from the metal backing in the event the laminate is bent to a particularly small radius. PTFE compounds also tend to be poor conductors of heat. As such, these bearings, even when fabricated using bronze filled PTFE sheets (see the discussion of "Norglide M" bearings hereinbelow), tend to exhibit relatively low heat transfer. Finally, although PTFE may be made electrically conductive, the hot melt film used to bond the bearing layer to the metal backing is electrically insulative. This aspect tends to make the Norglide bearings undesirable for use in applications that rely on electrical continuity such as, for example, electrostatic painting in the automotive and other metal fabricating industries.
The creep and delamination problems may be addressed by roughening the metal backing surface, such as by sandblasting, etc., prior to application of the bearing surface. However, the process is relatively cumbersome, time consuming and adds expense to the process.
As mentioned hereinabove, a variation of Norglide also available from Norton Pampus is known as "Norglide M." Norglide M is substantially similar to Norglide, but utilizes a bearing layer having an open-mesh metal fabric reinforcement disposed therein. The use of this reinforcement tends to ameliorate the aforementioned creep and conductivity drawbacks, but does not address the delamination and electrical conductivity concerns.
Thus, a need exists for an improved maintenance free bearing that addresses the problems of the prior art.